So Act 1, as mentioned, will be classic UT from Sans' perspective lore-wise. Sans will be slightly more emotional and impulsive, like how he lets Papyrus go confront you or how he reacts to Alphys and Asgore knowing more than they let on.

Lore time! Also text block so uh wait for the videos if you hate reading fr

Sans was snooping in Alphys' lab, as he often did, they were good friends, it didn't really matter, when he found an application called "Primarch.EXE" (omg exe so scary like sonk teh hogheg) curiousity gets the better of him and he runs it, seeing charts of lines labelled with everyone he knows, stopping, starting, jumping around. But not just one chart, many of them, labelled accordingly as "timelines".

He soon realises what this means, and leaves quickly. After that is when he makes his promise with Toriel. In the past, Sans would take a shortcut with the human, effectively skipping Snowdin altogether, but knowing what he knows, he decides to let Papyrus have his fun with the human, since they're harmless enough, and it might be safer.

In the past Sans would prevent them from meeting Paps cuz he knew the human's eventual fate and wanted to avoid the difficulty of explaining it all to Papyrus.

Anyway, he notices some things about the "human". Their expression, body language, their dusty clothes and knife. He goes to see the lady, ask em about the human, but they get no response, he eventually decides to enter the Ruins, weighing his concerns above breaching their privacy, and finds nothing but dust and a few cowering monsters, hiding rather than talking.

He has his suspicions unable to find the lady, coming to the obvious conclusion of their deaths. He goes to New Home, and convinces the administration to give him a census chart, given his position as judge, and he begins checking off monsters on the list who are alive/dead.

He realises the danger Papyrus is in, and sticks by him as he performs his puzzles. He checks of monsters around Snowdin too, and later, talks to Papyrus warning him about the human. Papyrus is insistent, and Sans eventually cedes, deciding it doesn't really matter, if time and space will be reset anyway. After Papyrus leaves he immediately realises what a stupid idea that was, and runs to stop him.

But he's too late. (oh noooooooooooo)

He picks up the scarf. Filled with a rush of mixed feelings. Should he care? Why does he care? They're all gonna die. He already knows everyone is gonna die. He saw it. They practically already did.

Despite that he can't help but break down. (he doesn't die skeletons don't have tear ducts duh) but he does eventually pick himself back up.

He follows the "human" checking off each monster killed, and eventually jumps ahead to talk to Alphys, he knows about the cameras. He knows she knows about the human. He doesn't want to let on about the Primarch.EXE. She probably knows. It was on her Computer after all, but he doesn't want to think about it.

Alphys is evacuating monsters when he finds her, and she lets on that she knows about the timelines. Despite knowing she knew, it really annoys Sans, because why hasn't she done anything to prevent this if she knows its happening? He goes to Asgore. Asgore also lets on that he knows. This annoys Sans even more, the KING knows but hasn't done anything despite having the most power in the Underground. And even now, still they do nothing.

Sans leaves, he knows he made a promise, but seeing all this, he can't afford not to care anymore. He holds a small funeral for Papyrus. He knows what he's about to do is stupid. It's a death sentence. But he needs to do SOMETHING. Even if it's hopeless and predetermined. He figures the "human" is the one who can reset everything, so maybe he can force them to. He is the judge after all. Knowing them, it will end in his defeat, but he'll just be back next timeline anyway.

(As the battle does go on, he loses hope more and more, eventually having the realisation this could have happened before. He could've tried the exact same thing before and died the same way. But the rest of his arc for this Act will be developed through the fight when I make it.)